Such a master disc is known from German Offenlegungsschrift 32.37.236 which corresponds substantially to UK Patent Application GB 2,109,270.
The master disc forms the starting point in the manufacture of optically readable information carriers. It is the first disc in which optically readable information is stored by using laser light which is modulated in conformity with the information to be recorded. In the above master disc, irradiation with modulated laser light causes the recording layer to be heated in the exposed areas as a result of absorption of said light. The dye present in the layer facilitates the conversion of light into heat. The heating-up causes the nitrocellulose binder to decompose, so that a pit (bit) is formed in the exposed area. The discs is rotated during the exposure process, while the modulated laser light beam is moved diametrically across the plate, so that a spiral-shaped track of bits is formed.
The bits can be optically read immediately after they have been formed. Thus has the advantage that the quality and correctness of the inscribed bits can be checked immediately. Such a master disc is termed a direct-effect master disc.
The master disc which has been frequently used in practice up to now comprises a glass substrate to which a layer of a photoresist is applied. After exposure of the photoresist layer to a modulated laser light beam, the photoresist is treated with a developing liquid, so that, dependent on the type of photoresist used, the layer dissolves at the exposed or unexposed areas, thereby forming pits (information bits). This has the disadvantage that the quality of the inscribed information bits can only be checked after the disc has been developed. An advantage of the master disc on the basis of a photoresist is that the pits formed are very well defined and have an excellent signal-noise ratio when they are optically read.
A satisfactory pit definition is very important. It has to be taken into account that a copy is manufactured from a master disc, mostly a metal copy, which is termed father disc. One or more copies are manufactured from said father disc, the mother discs, which in turn is (are) used to manufacture matrices. Said matrices are used in an injection molding or compression molding process to manufacture a large number of optically readable synthetic resin information carriers such as Compact Discs or Laser Discs. This entire replica process is based on the basis product, the master disc, so that said disc has to meet stringent quality requirements. In this connection, it is noted that the dimensions of the bits on a Compact Disc must meet accurate standard requirements. For example, the width dimension of the bits must be approximately 0.5 .mu.m. The distance between the turns of the spiral-shaped track is 1.6 .mu.m. The length dimensions of the bits are representative of the information stored and, on using an EFM-modulation must vary, in discrete steps of 0.3 .mu.m, between 0.9 and 3.6 .mu.m. This means, that it must be possible to provide (inscribe) and optically distinguish bits having length dimensions of 0.9; 1.2; 1.5; 1.8; 2.1; 2.4; 2.7; 3.0; 3.3 and 3.6 .mu.m.
The shape of the pit (bit definition), the signal-noise ratio when the bits are read and, in particular, the so-called block-error rate, which is a measure of the number of master-read errors, are important factors in the manufacture and use of the master.
The above-mentioned direct effect master based on a recording layer of nitrocellulose which comprises a dye, has the disadvantage that the shape of the pit obtained is not optimal and substantially less satisfactorily defined than the shape of the pit in a photoresist master. This means that the signal-noise ratio is unsatisfactory and that the block-error rate is increased. A further problem is that due to the decomposition of nitrocellulose and dye in the exposed areas, the reaction products formed in said process deposit on the surface of the master disc. This deposited residue is termed debris and causes a deterioration of the quality of the master disc.
The latter problem is mentioned as such in the above Offenlegungsschrift 32.37.236. According to said literature reference, better results are obtained when a binder of cellulose nitrate is used which is formed from a cellulose starting material comprising at least 97% of alpha cellulose.